Investigating Specific Cation Interactions with Anionic Surfactant Monolayers at the Air/Water Interface Using Nonlinear Spectroscopy and Thermodynamics
Restricted (Penn State Only)
- Author:
- Judd, Kenneth Duane
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 28, 2022
- Committee Members:
- Paul Cremer, Chair & Dissertation Advisor
Benjamin Lear, Major Field Member
Seong Kim, Outside Unit & Field Member
Mark Maroncelli, Major Field Member
Philip Bevilacqua, Program Head/Chair - Keywords:
- ion pairing
ion specific effects
monolayers
spectroscopy
nonlinear optics
vibrational spectroscopy
thermodynamics
surfactants - Abstract:
- Ion specific effects in aqueous solution are responsible for a wide range of phenomenon in biology, colloidal chemistry, industrial processes, and atmospheric chemistry. The origin of the study of ion specificity in aqueous solution was the work of Franz Hofmeister in 1888, who discovered that some salts precipitated protein better than others. As ubiquitous and important as ion specific effects are, a molecular level picture and unifying theory remains elusive. Continuum models developed to describe behavior in electrolytic solution ignore hydration water and treat ions as infinitesimally small point charges and are thus unable to account for ion specificity in real solution. In order for ion paring to occur, hydration water molecules must be shed from one or both ions involved in the pairing. Small and charge dense ions hold onto their hydration shell water molecules more tightly than larger ions of the same charge, while also forming stronger interaction with counterions via coulombic attraction. The Law of Matching Water Affinities (LMWA), introduced about 25 years ago, posited that ion pairing between oppositely charged ions is more favorable when the thermodynamic hydration properties are closely matched in value. This was based on the work of Fajans who came to the same conclusion in the 1920s. The LMWA extended Fajans’ ideas to include biologically relevant functional groups. Moreover, the LMWA claimed that ion pairing between oppositely charged ions of similar thermodynamic hydration would result in contact ion pairing, while water-mediated ion pairing would occur if there was a significant mismatch in the hydration of the cation and anion. While a clear trend exists between thermodynamic hydration properties and specific ion affinity in alkali halide salts, the structure of polyatomic functional groups introduces structural considerations which cannot be ignored. In this work, specific ion pairing at charged surfactant interfaces investigated the claims of the LMWA, focusing on two aspects. The first is that contact ion pairing would be expected when there is little mismatch in the thermodynamic hydration of the ions. Using vibrational sum frequency spectroscopy (VSFS) and pressure-area (Π-A) isotherms it was found that the weakly hydrated alkyl sulfate functional group of an eicosyl sulfate (ESO4) monolayer does indeed follow the order predicted by the LMWA, with binding decreasing in the order K+ > Na+ > Li+. However, these techniques are not sensitive to the nature of the ion pair and unable to differentiate water-mediated ion pairing from contact ion pairing. VSFS measurement of the symmetric stretching mode of the sulfate group revealed significant dehydration of the headgroup in the presence of Li+, but not K+. This is surprising, and not only because it shows that contact ion pair formation follows the opposite order predicted by the LMWA. It also reveals a system in which stronger binding does not correspond to a greater propensity for contact ion pair formation. In a second set of experiments the effect of intramolecular and intermolecular structure on the binding order predicted by the LMWA was investigated. Intramolecular structure was altered by switching the alkyl sulfate headgroup of an ESO4 monolayer out for the alkyl sulfonate headgroup of an octadecyl sulfonate (ODSO3) monolayer. The sulfate and sulfonate headgroups are both weakly hydrated, with similar thermodynamic hydration values. The chemical structure is different, however, with the sulfonate headgroup lacking the oxygen atom linking the SO3 group to the carbon tail. Intramolecular structure was changed by diluting ESO4 into an eicosanol monolayer. Spreading the sulfate headgroups apart in an alcohol monolayer solvent significantly changes the monolayer structure while maintaining the same functional group at the interface. VSFS spectra of the OH stretch region and Π-A isotherms showed a high degree of variation in the specific ion effects between the monolayers and alkali metal cations. Cs+ was found to have the greatest affinity to ESO4/eicosanol mixed monolayers, yet the weakest affinity to pure ODSO3 monolayers. Intermediate behavior was observed for pure ESO4 monolayers. These results clearly show the importance of chemical structure on specific ion binding and demonstrated that structure must be considered. Finally, the pKa of stearic acid (SA) at the interface with varying surface charge density was experimentally measured using VSFS. VSFS analysis of the symmetric stretching mode of monolayers consisting of SA and octadecanol in different proportions experimentally measured the pKa of SA in the different monolayer compositions. These experimental results were then compared to predictions of Poisson-Boltzmann (PB) theory and a modified Poisson-Boltzmann model (mPB) which includes a size parameter to account for the finite size of hydrated ions in solution. It was found that PB accurately predicts surface properties up to a surface charge density of 150 mC/m2, while the mPB model must be used at higher surface charge density. Nonetheless, the mPB model was found to hold good agreement with experiment up to the highest surface charge density of 320 mC/m2.